1. Field of the Invention
The present invention relates to a vehicle body frame structure for motorcycles, and more particularly to a vehicle body frame structure of the type that a single-tube main frame extending from a head pipe to the rear of the vehicle body and a single-tube down frame extending downwards from the head pipe are connected with each other via a plurality of frame members.
2. Description of the Prior Art
A vehicle body frame of a motorcycle is a basic structure of a vehicle having a role of carrying an engine, a power transmission, various equipments, a rider, a load and the like and connecting front and rear wheels with each other, and especially, since a front portion of the vehicle body frame is the most important part which supports an engine and a full tank and is connected to a steering head, sufficient strength and rigidity are required for the front portion of the vehicle body.
Hence, in the case of a high speed motor cycle or a racing motorcycle in which a heavy load is applied to the vehicle body frame, often a cradle type frame, that is, a frame having a shape similar to a cradle for carrying an engine is employed, and the cradle type frames are further classified into a double cradle type frame in which two down frame tubes connected to a head pipe are disposed in parallel to each other, and a semi-double cradle type frame in which a single-tube down frame connected to a head pipe is branched into two tubes in the midway.
However, especially in the case of a motorcycle for running on an irregular ground, for the purpose of stably maintaining a riding attitude by clamping a fuel tank with rider's both knees upon running on an irregular ground, the width of the fuel tank is made narrow, and consequently, it is necessary to narrow the gap space between inner side walls of a fuel tank having an inverse-U-shaped cross-section configuration to a minimum by employing the semi-double cradle type frame in which the main frame is formed of a single tube and thereby assure a fuel tank capacity. In other words, if the main frame is made narrow in width, then reasonably the capacity of the fuel tank can be increased. However, in order to assure the strength and rigidity of the vehicle body frame, the narrowing in width of the main frame would be naturally limited.